High temperature viscosifying and fluid loss controlling additives for well cements, well cement compositions and methods

ABSTRACT

A high temperature viscosifying and fluid loss controlling additive for use in well cements, well cement compositions including the additive and methods of using the compositions are provided. The well cement compositions of this invention are comprised of a hydraulic cement, water and a viscosifying and fluid loss controlling additive comprising a mixture of a polymer comprised of at least one monomer which is calcium tolerant, anionic and disperses basic cement slurries, at least one monomer which hydrolyzes in basic cement slurries to generate anionic carboxylate groups that bind with calcium and viscosify the slurries and at least one monomer which generates non-ionic pendant groups on the polymer upon hydrolyzing in basic cement slurries to prevent polymer precipitation and a homopolymer of a monomer which hydrolyzes in basic cement slurries to generate anionic carboxylate groups that bind with calcium, viscosify the slurries and prevent settling in the slurries.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of application Ser. No. 10/071,076filed on Feb. 8, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to subterranean well cementing operations,and more particularly, to viscosifying and fluid loss controllingadditives for use in well cements subjected to temperatures up to 500°F., well cement compositions containing the additives and methods ofusing the compositions.

2. Description of the Prior Art

Hydraulic cement compositions are commonly utilized in subterranean wellcompletion and remedial operations. For example, hydraulic cementcompositions are used in primary cementing operations whereby strings ofpipe such as casings and liners are cemented in well bores. Inperforming primary cementing, a hydraulic cement composition is pumpedinto the annular space between the walls of a well bore and the exteriorsurfaces of a pipe string disposed therein. The cement composition ispermitted to set in the annular space thereby forming an annular sheathof hardened substantially impermeable cement therein which supports andpositions the pipe string in the well bore and bonds the exteriorsurfaces of the pipe string to the walls of the well bore. Hydrauliccement compositions are also utilized in remedial cementing operationssuch as plugging highly permeable zones or fractures in well bores,plugging cracks or holes in pipe strings and the like.

Cement composition viscosifying additives are often used in well cementcompositions to prevent settling of solids in the cement compositionsafter they are placed in a subterranean zone to be cemented. While suchcement composition viscosifying additives have been utilizedsuccessfully at subterranean temperatures up to about 350° F., at highertemperatures the heretofore utilized viscosifying additives have notbeen able to prevent thermal thinning which results in the settling ofsolids in cement compositions. The settling of the solids in a cementcomposition results in defective cementing and failure of the set cementto provide zonal isolation.

Fluid loss control agents are also used in well cement compositions toreduce fluid loss from the cement compositions to permeable formationsor zones into or through which the cement compositions are pumped. Inprimary cementing, the loss of fluid, i.e., water, to permeablesubterranean formations or zones can result in premature gelation of thecement composition whereby bridging of the annular space between thepermeable formation or zone and the pipe string being cemented thereinprevents the cement composition from being placed over the entire lengthof the annulus.

Thus, there are needs for an improved viscosifying additive for wellcements which can be utilized in cements subjected to temperatures up to500° F. and which also provides fluid loss control to the cements atsuch temperatures, improved well cement compositions containing theadditive and methods of using the cement compositions.

SUMMARY OF THE INVENTION

The present invention provides an improved viscosifying additive for usein well cement compositions subjected to temperatures up to 500° F.which also provides fluid loss control to the cement compositions, wellcement compositions including the additive and methods of using thecement compositions which meet the needs described above and overcomethe deficiencies of the prior art.

The viscosifying and fluid loss controlling additive of this inventionfor use in well cements at temperatures up to 500° F. is basicallycomprised of a mixture of a polymer and a homopolymer. The polymer iscomprised of at least one monomer which is calcium tolerant, anionic anddisperses basic cement slurries, at least one monomer which hydrolyzesin basic cement slurries to generate anionic carboxylate groups thatbind with calcium and viscosify the slurries and at least one monomerwhich generates non-ionic pendant groups on the polymer upon hydrolyzingin basic cement slurries to prevent polymer precipitation, and ahomopolymer of a monomer which hydrolyzes in basic cement slurries togenerate anionic carboxylate groups that bind with calcium, viscosifythe slurries and prevent settling in the slurries. The polymerpreferably has a molecular weight in the range of from about 300,000 toabout 1.5 million and the homopolymer preferably has a molecular weightin the range of from about 900,000 to about 1.5 million. The polymer ispresent in the mixture in an amount in the range of from about 50% toabout 95% by weight of the mixture and the homopolymer is presenttherein in an amount in the range of from about 5% to about 50% byweight of the mixture.

The improved well cement compositions of this invention are basicallycomprised of a hydraulic cement, water present in an amount sufficientto form a slurry and a viscosifying and fluid loss controlling additivecomprised of a mixture of a polymer and a homopolymer as describedabove.

The viscosifying and fluid loss controlling additive can be added inparticulate solid form directly to the hydraulic cement or mix waterutilized or it can be combined with water whereby a storable aqueoussolution is formed which can be conveniently combined with the cementcomposition mix water.

The methods of this invention for cementing a zone in a subterraneanformation penetrated by a well bore are basically comprised of the stepsof preparing a cement composition of this invention including aviscosifying and fluid loss controlling additive comprised of a mixtureof a polymer and a homopolymer as described above, placing the cementcomposition in the zone to be cemented and allowing the cementcomposition to set into an impermeable solid mass therein.

It is, therefore, a general object of the present invention to providean improved high temperature viscosifying and fluid loss controllingadditive for well cements, well cement compositions and methods of usingthe cement compositions.

Other and further objects, features and advantages of the presentinvention will be readily apparent to those skilled in the art upon areading of the description of preferred embodiments which follows.

DESCRIPTION OF PREFERRED EMBODIMENTS

The viscosifying and fluid loss controlling additive of this inventionis basically comprised of a mixture of a polymer and a homopolymer. Thepolymer is characterized in that it is obtained by polymerizing: (I) atleast one monomer which is calcium tolerant, anionic and disperses basiccement slurries selected from 2-acrylamido-2-methylpropane sulfonic acidand its salts; vinyl sulfonate, allyl sulfonate or3-allyloxy-2-hydroxy-1-propanesulfonic acid and its salts; (2) at leastone monomer which is capable of hydrolyzing in basic cement slurries togenerate anionic carboxylate groups that bind with calcium, viscositythe slurries and prevent settling in the slurries selected fromacrylonitrile, acrylamide, N,N-dialkylacrylamide wherein the alkyl groupis selected from C₁ to C₆ alkyl groups, N-vinylpyrrolidone,2-acrylamido-2-methylpropane sulfonic acid and its salts oralkylacrylates such as methylmethacrylate and methylacrylate; and (3) atleast one monomer which generates non-ionic pendant groups on thepolymer upon hydrolysis in basic cement slurries to prevent polymerprecipitation selected from N-alkyl-N-vinylalkanamides such asN-methyl-N-vinylacetamide, allyl glycidyl ether or vinylacetate. Themolecular weight of the polymer is advantageously in the range of fromabout 300,000 to about 1.5 million, preferably about 500,000.

The ration of the monomers in the polymer is selected such that whenpresent in cement slurries, the polymer does not viscosify the slurriesexcessively under ambient conditions, but upon undergoing hydrolysisreactions in the cement slurries during placement, the polymer willcontinuously generate sufficient carboxylate groups at down holetemperatures to react with the calcium ions present in the slurries incross-linking reactions and viscosify the slurries to counteract thethermal thinning of the slurries. Thus, the higher the down holetemperature, the greater the thermal thinning of cement slurries and thegreater the rate of slurry viscosification due to carboxylate generatinghydrolysis reactions.

The polymer, due to its adsorption on cement grains as well as due toits fluid viscosification properties, also provides fluid losscontrolling properties. When the fluid loss control of slurriescontaining the polymer is not adequate, additional fluid losscontrolling polymers can be added.

The preferred monomer ratio in the polymer depends on how the polymeraffects the viscosities of cement slurries under ambient conditions aswell as under down hole conditions. High viscosities under ambientconditions result in excessive pump pressures during the placement ofthe slurries. On the other hand, excessive dispersion of cement slurriesat ambient temperature causes particle settling even before the slurriesare placed in the well. The preferred ratio of the dispersing monomer ormonomers from group 1 set forth above may vary from 30 to 60 percent byweight of the polymer; the anionic carboxylate generating monomer ormonomer(s) of group 2 above may range from 20 to 60 percent by weight ofthe polymer; and the monomer or monomers from group 3 above may rangefrom 0 to 40 percent by weight of the polymer. Most preferably, themonomer or monomers from group 1 are in the 40-50 percent range byweight of the polymer; the monomer or monomers of group 2 are present inthe 30-40 percent range; and the group 3 monomer or monomers are in the10-20 percent range by weight of the polymer.

It has been found that different homopolymers containing carboxylategenerating monomers at different temperatures can be used in combinationwith the above described polymer. For example, it has been found thatthe polymer of the present invention can be used in a synergisticmixture with other homopolymers obtained by polymerizing one or more ofthe monomers listed in group 2 above. Thus for example, the abovedescribed polymer can advantageously be used in admixture withpolyvinylpyrrolidone or polyacrylamide for preventing particle settlingin cement slurries. The polymer is generally present in the mixture inan amount in the range of from about 50% to about 95% by weight of themixture and the homopolymer or homopolymers used are present therein inan amount in the range of from about 5% to about 50% by weight of themixture.

Viscosifying and fluid loss controlling polymers useful in accordancewith this invention are available commercially. For example, polymershaving the trade names “HOSTAMER V 4707™”, “HOSTAMER 4706™” and“HOSTADRILL 2825™” are commercially available from the ClariantCorporation of Charlotte, N.C., and contain 2-acrylamido-2-methylpropanesulfonic acid, acrylamide and N-vinyl-N-alkylalknamide in proprietaryratios and are described in U.S. Pat. No. 4,587,283 issued to Hille etal. on May 6, 1986 which is incorporated herein by reference. A polymercontaining multiple group 2 monomers with different hydrolysis rates isavailable under the trade designation “HE 300™” from DrillingSpecialties Company of Bartlesville, Okla. The “HE 300™” polymercontains 2-acrylamido-2-methylpropane sulfonic acid, N-vinylpyrrolidoneand acrylamide in a proprietary ratio. A homopolymer of vinylpyrrolidonehaving a molecular weight in the range of from about 900,000 to about1.5 million is commercially available under the trade designation of“PVP K-90™” from ISP Technologies Incorporated of Wayne, N.J.

The viscosifying and fluid loss controlling additive of this inventioncan be combined in a solid particulate form with the hydraulic cement ormix water utilized to form a well cement composition. When the additiveis utilized in offshore well cementing operations, the additive ispreferably in liquid form. That is, the solid particulate polymerdescribed above with at least one homopolymer described above can becombined with water in an amount sufficient to form an aqueous solutionof the additive. The water used can be fresh water or salt water. Thepolymer and homopolymer mixture can be used effectively in combinationwith a separate fluid loss controlling additive as well as other wellcement composition additives.

Thus, a preferred viscosifying and fluid loss controlling additive ofthis invention for use in well cement compositions subjected totemperatures up to 500° F. is comprised of a mixture of a polymercomprised of at least one monomer which is calcium tolerant, anionic anddisperses basic cement slurries, at least one monomer which hydrolyzesin basic cement slurries to generate anionic carboxylate groups thatbind with calcium and viscosify the slurries and at least one monomerwhich generates non-ionic pendant groups on the polymer upon hydrolyzingin basic cement slurries to prevent polymer precipitation and ahomopolymer of a monomer which hydrolyzes in basic cement slurries togenerate anionic carboxylate groups that bind with calcium, viscositythe slurries and prevent settling in the slurries.

The improved well cement compositions of the present invention arebasically comprised of a hydraulic cement, sufficient water to form aslurry and a viscosifying and fluid loss controlling additive of thepresent invention as described above.

A variety of hydraulic cements can be utilized in accordance with thepresent invention including those comprised of calcium, aluminum,silicon, oxygen and/or sulfur which set and harden by reaction withwater. Such hydraulic cements include, but are not limited to, Portlandcements, pozzolana cements, gypsum cements, aluminous cements, silicacements and alkaline cements. Portland cements are generally preferredfor use in accordance with the present invention. Portland cements ofthe types defined and described in API Specification For Materials AndTesting For Well Cements, API Specification 10, 5^(th) Edition, datedJul. 1, 1990 of the American Petroleum Institute are particularlypreferred. API Portland cements include Classes A, B, C, G and H. APIClasses G and H are preferred with Class G being the most preferred.

The water utilized in the cement compositions of this invention can befresh water, unsaturated salt solutions including brines and seawaterand saturated salt solutions. Generally, the water can be from anysource provided it does not contain an excess of compounds thatadversely affect other components in the cement compositions. The wateris present in the cement compositions of this invention in an amountsufficient to form a pumpable slurry. More particularly, the water ispresent in the cement compositions in an amount in the range of fromabout 38% to about 70% by weight of hydraulic cement therein, morepreferably in an amount of about 60%.

The viscosifying and fluid loss controlling additive of this inventionis included in the cement compositions of this invention in an amount inthe range of from about 0.2% to about 7% by weight of hydraulic cementtherein, more preferably in an amount in the range of from about 0.5% toabout 3% and most preferably in an amount of 2%.

As mentioned above, the cement compositions containing the viscosifyingand fluid loss controlling additive of this invention maintainviscosities at levels sufficient to prevent significant settling ofparticulate solids in the cement compositions at temperatures up to ashigh as 500° F. In addition, the additive provides fluid loss control tothe cement compositions which often eliminates the need for one or moreseparate fluid loss control additives in the cement compositions.

As is well understood by those skilled in the art, various other cementcomposition additives can be utilized in the cement compositions of thisinvention including, but not limited to, additional fluid losscontrolling agents, set retarding agents, set accelerating agents,fillers, weighting materials and the like.

A preferred well cement composition of this invention for use attemperatures up to about 500° F. is comprised of: a hydraulic cement;water present in an amount sufficient to form a slurry; and aviscosifying and fluid loss controlling additive comprised of a mixtureof a polymer formed of at least one monomer which is calcium tolerant,anionic and disperses basic cement slurries, at least one monomer whichhydrolyzes in basic cement slurries to generate anionic carboxylategroups that bind with calcium and viscosify the slurries and at leastone monomer which generates non-ionic pendant groups on the polymer uponhydrolyzing in basic cement slurries to prevent polymer precipitationand a homopolymer of a monomer which hydrolyzes in basic cement slurriesto generate anionic carboxylate groups that bind with calcium, viscosifythe slurries and prevent settling in the slurries.

A more preferred well cement composition of this invention for use attemperatures up to about 500° F. is comprised of: a hydraulic cement;water present in an amount sufficient to form a slurry; and aviscosifying and fluid loss controlling additive having a molecularweight of about 500,000 comprised of a mixture of a polymer formed of2-acrylamido-2-methylpropane sulfonic acid, acrylamide andN-alkyl-N-vinyl-acetamide monomers, the 2-acrylamido-2-methyl propanesulfonic acid monomer being present in the polymer in an amount in therange of from about 40% to about 50% by weight of the polymer, theacrylamide monomer being present in the polymer in an amount in therange of from about 30% to about 40% by weight of the polymer and theN-alkyl-N-vinyl-acetamide being present in an amount in the range offrom about 10% to about 20% by weight of the polymer and a homopolymerof acrylamide having a molecular weight of about 1 million.

The methods of the present invention for cementing a subterranean zonepenetrated by a well bore are basically comprised of the steps ofproviding a cement composition comprised of a hydraulic cement, waterpresent in an amount sufficient to form a slurry and a viscosifying andfluid loss controlling additive comprised of a mixture of a terpolymerand a homopolymer as described above, placing the cement composition inthe subterranean zone to be cemented and allowing the cement compositionto set into an impermeable solid mass therein.

A preferred method of this invention for cementing a subterranean zonepenetrated by a well bore is comprised of the steps of: (a) providing acement composition comprising a hydraulic cement, water present in anamount sufficient to form a slurry and a viscosifying and fluid losscontrolling additive comprised of a mixture of a polymer formed of atleast one monomer which is calcium tolerant, anionic and disperses basiccement slurries, at least one monomer which hydrolyzes in basic cementslurries to generate anionic carboxylate groups that bind with calciumand viscosify the slurries and at least one monomer which generatesnon-ionic pendant groups on the polymer upon hydrolyzing in basic cementslurries to prevent polymer precipitation and a homopolymer of a monomerwhich hydrolyzes in basic cement slurries to generate anioniccarboxylate groups that bind with calcium, viscosity the slurries andprevent settling in the slurries; (b) placing the cement composition inthe subterranean zone; and (c) allowing the cement composition to settherein.

A more preferred method of this invention for cementing a subterraneanzone penetrated by a well bore is comprised of the steps of: (a)providing a cement composition comprising a hydraulic cement, waterpresent in an amount sufficient to form a slurry and a viscosifying andfluid loss controlling additive having a molecular weight of about500,000 comprised of a mixture of a polymer formed of2-acrylamido-2-methylpropane -sulfonic acid, acrylamide andN-alkyl-N-vinyl-acetamide monomers, the 2-acrylamido-2-methyl propanesulfonic acid monomer being present in the polymer in an amount in therange of from about 40% to about 50% by weight of the polymer, theacrylamide monomer being present in the polymer in an amount in therange of from about 30% to about 40% by weight of the polymer and theN-alkyl-N-vinyl-acetamide being present in an amount in the range offrom about 10% to about 20% by weight of the polymer and a homopolymerof acrylamide having a molecular weight of about 1 million; (b) placingthe cement composition in the subterranean zone; and (c) allowing thecement composition to set therein.

In order to further illustrate the viscosifying and fluid losscontrolling additive, well cement compositions and methods of thepresent invention, the following example is given.

EXAMPLE

Tests were conducted on well cement compositions containing the hightemperature viscosifying and fluid loss controlling additive of thisinvention.

Cement Slurry No. 1 contained Portland Class A cement as the basicingredient. The other ingredients present in Slurry No. 1 are shown inTable I. Cement Slurries Nos. 2 through 23 contained Portland Class Hcement, silica flour in an amount of 35% by weight of cement and theother ingredients shown in Tables I and II. The basic cement slurriesalso contained viscosity increasing and settling preventing polymers asshown in Tables I and II.

Cement Slurries Nos. 24 through 33 contained Portland Class H cement,40% silica flour by weight of cement and 5% fumed silica by weight ofcement. The other additives in Cement Slurries Nos. 24 through 33 arelisted in Table III. The cement retarders used in the tests arecommercially available from Halliburton Energy Services of Duncan, Okla.under the trade names “FDP 601™” (lignosulfonate retarder), “SCR-100™”(a copolymer of 2-acrylamide-2-methylpropane sulfonic acid and acrylicacid), “SCR-500™” (a copolymer of 2-acrylamido-2-methylpropane sulfonicacid and itaconic acid), “HR-15™” (a mixture of lignosulfonate andtartaric acid), and “HR-5™” (sodium salt of lignosulfonate). The fluidloss control agents used are also commercially available as Halliburtonproducts, namely “Halad 413™” (causticized lignite grafted with2-acrylamido-2-methylpropane sulfonic acid, N,N-dimethylformamide andacrylonitrile) and “Halad 344™” (a copolymer of N,N-dimethylformamideand 2-acrylamido-2-methylpropane sulfonic acid). The transition timeincreasing additive, “GasStop HT™” (tannin grafted with acrylamide and2-acrylamido-2-methylpropane sulfonic acid) is also available fromHalliburton as is the settling preventing additive “SA-541™”(causticized hydroxypropyl guar surface treated with sodium borate).

Slurries Nos. 1 through 10 were tested at temperatures in the range offrom 150° F. to 250° F. Slurries Nos. 11 through 23 were tested attemperatures in the range of from 300° F. to 400° F. and Slurries Nos.24 through 33 were tested at a temperature of 434° F.

The viscosity, variation in density, set time, fluid loss and rheologytests were conducted utilizing the following procedures. All the testswere carried out on an instrument available from Halliburton EnergyServices, Inc. under the trade name “MINI-MACS™.” The “MINI-MACS™”instrument is capable of measuring the consistency (Bc or Beardenunits), the viscosity, and the static gel strength of a cement slurry. Avariable speed stepper motor drive and precision force transducer isused for stirring the slurry and measuring the consistence and staticgel strength values. The motor speed is controlled to rotate a paddleinside the slurry container at 150 rpm for a standard API thickeningtime test and the initial placement conditioning for a static gelstrength test. When in the static gel strength testing mode, the paddleis rotated at 0.2 degrees per minute.

The MINI-MACS™” instrument is rated for use at pressures up to 20,000psi and temperatures up to 500° F. A heating rate of approximately 10°F./min is attainable. The test chamber is a two-part assembly thatcontains flow channels between the inside and outside portions that areused for cooling. This results in extremely fast cooling of theinstrument after a testing is completed, even for high temperaturetests. The API thickening time test provides the user with informationon how long a slurry will remain pumpable during a cementing job. Theinstrument is one of the alternate apparatus for well simulationthickening time tests described in API RP10B, Twenty-Second Edition,December 1997, Appendix D. The instrument rotates the paddle inside theslurry cup instead of rotating the slurry cup as the traditionalconsistometer does. The static gel strength test is used to determinethe gellation properties of a slurry when in a static mode. The test isnormally started by stirring the slurry to simulate placement into awell bore and then the stirring stopped and placed in the static gelstrength mode. During this time, the paddle is rotated at 0.2 degreesper minute (as near to static conditions as practical to obtain anindication of gellation).”

The tests were done in the thickening time test mode. If the slurryviscosity dropped to zero Bc long before the thickening time, the slurrywas assumed to be vulnerable to settling. If a viscosity value of lessthan at least 5-10 Bc was maintained from the time the test temperatureis reached until the set time, then the slurry was not expected to havesettling problems. Frequently, it was noted that during the heatingcycle, especially with gums surface-treated for delayed hydration, theslurry viscosity suddenly increased to values which are considered toohigh for pumping and dropped to near zero values after a few minut4s athigh temperatures. In order for the slurry to be placed with reasonablepump pressures which do not exceed fracture gradients of the formation,slurries which maintain uniform viscosity through the entire length ofslurry placement are desired. During the course of the experiment using“MINI-MACS™”, Off/On cycles were programmed into the instrument computerprogram. During Off/On experiments, the stirring was stopped and startedagain in 5 minutes, and the cycle was repeated. When the stirring wasstarted again, if the immediate viscosity was higher than that when thestirring was stopped, it was taken as an indication settling or severegellation. If the immediate viscosity was the same as when the stirringwas stopped, it was taken as a good sign indicating that the slurry wasviscous and still flowable.

For selected slurries, the densities of different portions of set cementobtained from “MINI-MACS™” testing were measured to estimate the amountof settling. The fluid loss was measured either using a stirringhigh-temperature dynamic fluid loss cell for temperatures exceeding 180°F., or using the procedure described in API Specification 10, 5^(th)Edition dated Jul. 1, 1990 of the American Petroleum Institute. Therheology of the cement slurries was measured at room temperature using aFann viscometer Model 35 manufactured by Fann instruments of Houston,Tex.

A typical procedure is as follows for Slurry No. 30. A dry blend ofClass H cement (600 grams), tannin grafted with acrylamide and2-acrylamido-2-methylpropane sulfonic acid (4.8 grams), a copolymer of2-acrylamido-2-methylpropane sulfonic acid and itaconic acid (12 grams),tartaric acid (12 grams), silica flour (240 grams), fumed silica (30grams), polyvinylpyrrolidone (3 grams) and a terpolymer of2-acrylamido-2-methylpropane sulfonicacid/acrylamide/N-vinyl-N-methylacetamide (3 grams) was mixed with tapwater (372 grams) in a Waring blender under high shear according to APIprocedure. The slurry was transferred to the cell in the “MINI-MACS™”instrument and the heating cycle was set such that the test temperature(434° F.) was reached in 54 min. The stirring speed was 150 rpm. Thefinal pressure at this temperature was 15,000 psi. After the testtemperature was reached, the slurry viscosity was stabilized at 6 Bc.The slurry was allowed to set inside the cell at the test temperature.The device was cooled, the cement column was removed and chunks of thecement were removed with a drill. The densities of cement chunks fromtop, middle and bottom of the column were measured and were found to be15.45, 15.33 and 15.32, respectively. TABLE I Viscosity, Set Time, FluidLoss And Rheology At 150° F.-250° F. Range Cement Composition ComponentsAdditive Water, of the Present Invention Fluid Loss Lowest % byPolymer¹, Homopolymer², Control Additive³ Viscosity Set Fluid RheologySlurry wt. of % by wt. % by wt. % by wt. Set Retarders Temp., BeforeTime, Loss, At 300- No. cement of cement of cement of cement A⁴ B⁵ C⁶ D⁷° F. Set, Bc Hrs:Min cc 200-100-6-3 1 54.08 0.4 — 0.4 0.4 — — — 150 — —12 240-170-100- 12-8 2 54.40 1.0 — 1.0 — — — — 150 30 2:20 n.d.⁸ n.d. 354.24 1.0 — 1.0 0.2 — — — 150 25 9:30 n.d. 550-420-240- 24-16 4 54.241.0 — 1.0 0.2 — — — 200 11 6:11 n.d. n.d. 5 54.55 1.0 — — 0.2 — — — 20010 8:13 n.d. n.d. 6 54.24 1.0 — 1.0 0.2 — — — 200 18 11:06  n.d. n.d. 754.00 1.0 — 1.0 0.5 — 0.1 — 250 18 1:55 n.d. n.d. 8 54.08 1.0 — 1.0 0.37— 0.23 — 250 7 6:49 n.d. n.d. 9 53.92 1.0 — 1.0 0.6 — 0.1 — 250 9 3:10n.d. n.d. 10 54.22 1.0 — 1.0 — — — 0.6 250 16 5:41 n.d. n.d.¹2-acrylamido-2-methylpropane sulfonicacid/acrylamide/N-vinyl-N-alkylalknamide polymer²polyvinylpyrrolidone³causticized lignite grafted with 2-acrylamido-2-methylpropane sulfonicacid, N,N-dimethylformamide and acrylonitrile⁴Set Retarder A is lignosulfonate⁵Set Retarder B is a copolymer of 2-acrylamido-2-methylpropane sulfonicacid and acrylic acid⁶Set Retarder C is tartaric acid⁷Set Retarder D is lignosulfonate and tartaric acid⁸n.d. means not determined

The results in Table I show that in the 150-200° F. temperature range,the terpolymer containing acrylamide is sufficient to prevent settlingas seen in the good viscosity values of the slurry throughout the testperiod at the test temperatures. TABLE II Viscosity, Set Time, SetCement Density Variation And Rheology At 300° F.-400° F. Range CementComposition Components Additive Fluid Loss Variation Water, of thePresent Invention Control Lowest in Density Rheology % by Polymer¹,Homopolymer², Additive³ Viscosity Set of Set At Slurry wt. of % by wt. %by wt. % by wt. Set Retarders Temp., Before Time, Cement, 300-200-100-No. cement of cement of cement of cement A⁴ B⁵ C⁶ D⁷ ° F. Set, BcHrs:Min ppg 6-3, rpm 11 53.58 1.25 — 1.0 1.0 — 0.4 — 300 3 11:21  0.27600-540-328-40- 28 12 53.98 1.25 — 1.0 0.5 — 0.25 — 300 10 3:47 0.37600-480-300-44- 30 13 53.98 1.25 — 1.0 0.5 — 0.3 — 300 16 4:41 0.17⁹570-420-260-40- 24 14 53.98 1.25 — 1.0 — 1.0 0.4 — 300 10 5:05 —520-400-236-30- 20 15 53.98 — 1.25 1.0 — 1.0 0.4 — 300 7 4:37 0.51460-340-216-24- 16 16 53.76 — 1.25 1.0 — 1.5 0.75 — 300 2 15:23  — — 1754.17 1.25 — 1.0 — — — 0.8 300 15 2:30 — — 18 54.08 1.25 — 1.0 — — — 1.1300 12 5:18 0.37 — 19 54.01 1.56 — 1.25 — — — 1.1 300 21 7:01 — — 2054.08 — 1.25 1.0 — — — 1.1 300 0 6:54 — — 21 54.40 1.25 1.0  — — — — 1.1300 9 6:32 — — 22 53.68 1.56 — 1.25 — — — 2.25 400 14 2:04 — — 23 53.761.56 — 1.25 — 1.75⁸ — — 400 16 4:21 — —¹2-acrylamido-2-methylpropane sulfonicacid/acrylamide/N-vinyl-N-alkylalknamide polymer²polyvinylpyrrolidone³causticized lignite grafted with 2-acrylamido-2-methylpropane sulfonicacid, N,N-dimethylformamide and acrylonitrile⁴Set Retarder A is lignosulfonate⁵Set Retarder B is a copolymer of 2-acrylamido-2-methylpropane sulfonicacid and acrylic acid⁶Set Retarder C is tartaric acid⁷Set Retarder D is lignosulfonate and tartaric acid⁸Set Retarder B is 2-acrylamido-2-methylpropane sulfonic acid anditaconic acid instead of a copolymer of 2-acrylamido-2-methylpropanesulfonic acid and acrylic acid⁹No free water, settling streaking or particle separation was detected

The results in Table II for the temperature range 250-400° F. show thatthe polymers tested prevent settling of the particles as reflected inonly small density variation in the set cement. In the temperaturerange, the homopolymer of vinylpyrrolidone was inadequate when used byitself as seen by the zero viscosity value reached during the test. Thisis believed to be due to thermal thinning at intermediate temperaturesprior to reaching the test temperature. When the homopolymer is used incombination with the terpolymer containing the easily hydrolyzableacrylamide, the slurry viscosities remained at levels at which settlingdoes not occur. The results also show that by using a proper amount ofthe terpolymer in combination with additives which are not overlydispersing, the particulate settling can be effectively prevented. TABLEIII Viscosity, Set Time And Set Cement Density Variation At 434° F.Fluid Transition Variation Additive of the Present Invention Loss TimeSettling In Homo- Control Increasing Preventing Lowest Density Water, %Polymer¹, Polymer², polymer³, Additive⁴, Additive⁵, Additive⁶, ViscositySet Of Set Slurry by wt. % by wt. % by wt. % by wt. % by wt. % by wt. %by wt. Set Retarders Before Time, Cement, No. of cement of cement ofcement of cement of cement of cement of cement B⁷ C⁸ E⁹ Set, Bc Hr:MinPpg 24 62 — — — 1.2 0.8 0.5 1.0 0.5 — 0 — — 25 62 1.0 — — 1.2 0.8 0.52.0 — 0.2 0 — — 26 62 — 1.0 — — 0.8 — 2.0 2.0 — 3  9:00 — 27 62 — — 1.0— — — 2.0 2.0 — 0  7:20 — 28 62 1.0 — 1.0 — — — 2.0 2.0 — 0 26:00 — 2962 0.5 — 0.5 — — — 2.0 2.0 — 5  2:20 — 30 62 — 1.0 1.0 — — — 2.0 2.0 —1 >7:00 — 31 62 — 0.5 0.5 — — — 2.0 2.0 — 6  2:30 0.3 32 62 0.5 0.5 — —0.8 — 2.0 2.0 — 2 — 1.1 33 62 — 1.0 1.0 — 0.8 — 2.0 2.0 — 5  7:00 0.22¹2-acrylamido-2-methylpropane sulfonicacid/N-vinylpyrrolidone/acrylamide polymer²2-acrylamido-2-methylpropane sulfonicacid/acrylamide/N-vinyl-N-methylacetamide polymer³Polyvinylpyrrolidone⁴Causticized lignite grafted with 2-acrylamido-2-methylpropane sulfonicacid, N,N-dimethylformamide and acrylonitrile⁵Tannin grafted with acrylamide and 2-acrylamido-2-methylpropanesulfonic acid⁶Causticized hydroxypropylguar surface treated with sodium borate⁷Set Retarder B is a copolymer of 2-acrylamido-2-methylpropane sulfonicacid and acrylic acid⁸Set Retarder C is tartaric acid⁹Copolymer of 2-acrylamido-2-methylpropane sulfonic acid and acrylicacid

The results in Table III show that the proper combination of polymerswhich generate carboxylate groups by hydrolysis continuously over theentire temperature range will provide the slurry viscosificationnecessary to prevent particle settling at the test temperature.

Thus, the present invention is well adapted to carry out the objects andattain the ends and advantages mentioned as well as those which areinherent therein. While numerous changes may be made by those skilled inthe art, such changes are encompassed within the spirit of thisinvention as defined by the appended claims.

1. A viscosifying and fluid loss controlling additive comprising amixture of a polymer comprised of at least one monomer which is calciumtolerant, anionic and disperses basic cement slurries, at least onemonomer which hydrolyzes in basic cement slurries to generate anioniccarboxylate groups that bind with calcium and viscosify the slurries andat least one monomer which generates non-ionic pendant groups on thepolymer upon hydrolyzing in basic cement slurries to prevent polymerprecipitation and a homopolymer of a monomer which hydrolyzes in basiccement slurries to generate anionic carboxylate groups that bind withcalcium, viscosity the slurries and prevent settling in the slurries. 2.The viscosifying and fluid loss controlling additive of claim 1 whereinsaid monomer which is calcium tolerant, anionic and disperses basiccement slurries is selected from the group consisting of2-acrylamido-2-methylpropane sulfonic acid and its salts; vinylsulfonate, allylsulfonate and 3-allyloxy-2-hydroxy-1-propane sulfonicacid and its salts.
 3. The viscosifying and fluid loss controllingadditive of claim 1 wherein said monomer which is calcium tolerant,anionic and disperses basic cement slurries is2-acrylamido-2-methylpropane sulfonic acid or its salts.
 4. Theviscosifying and fluid loss controlling additive of claim 1 wherein saidmonomer which hydrolyzes in basic cement slurries to generate anioniccarboxylate groups which bind with calcium and viscosify the slurries isselected from the group consisting of acrylonitrile, acrylamide,N,N-dialkylacrylamide wherein the alkyl group is selected from C₁ to C₆alkyl groups, N-vinylpyrrolidone, 2-acrylamido-2-methylpropane sulfonicacid and its salts, alkylacrylate wherein the alkyl group is selectedfrom C₁ to C₆ alkyl groups and alkylmethylacrylate wherein the alkylgroup is selected from C₁ to C₆ alkyl groups.
 5. The viscosifying andfluid loss controlling additive of claim 1 wherein said monomer whichhydrolyzes in basic cement slurries to generate anionic carboxylategroups which bind with calcium and viscosify the slurries is acrylamide.6. The viscosifying and fluid loss controlling additive of claim 1wherein said monomer which generates non-ionic pendant groups on thepolymer upon hydrolyzing in basic cement slurries to prevent polymerprecipitation is selected from the group consisting ofN-alkyl-N-vinyl-acetamide wherein the alkyl group is selected from C₁ toC₆ alkyl groups, allyl glycidyl ether and vinylacetate.
 7. Theviscosifying and fluid loss controlling additive of claim 1 wherein saidmonomer which generates non-ionic pendant groups on the polymer uponhydrolyzing in basic cement slurries to prevent polymer precipitation isN-alkyl-N-vinyl-acetamide.
 8. The viscosifying and fluid losscontrolling additive of claim 1 wherein said monomer which is calciumtolerant, anionic and disperses basic cement slurries is present in saidpolymer in an amount in the range of from about 30% to about 60% byweight of said polymer.
 9. The viscosifying and fluid loss controllingadditive of claim 1 wherein said monomer which hydrolyzes in basiccement slurries to generate anionic carboxylate groups which bind withcalcium and viscosify the slurries is present in said polymer in anamount in the range of from about 20% to 60% by weight of said polymer.10. The viscosifying and fluid loss controlling additive of claim 1wherein said monomer which generates non-ionic pendant groups on thepolymer upon hydrolyzing in basic cement slurries to prevent polymerprecipitation is present in said polymer in an amount in the range offrom about 0 to 40% by weight of said polymer.
 11. The viscosifying andfluid loss controlling additive of claim 1 wherein said polymer ispresent in said mixture of said polymer and said homopolymer in anamount in the range of from about 50% to about 95% by weight of saidmixture and said homopolymer is present therein in an amount in therange of from about 5% to about 50% by weight of said mixture.
 12. Theviscosifying and fluid loss controlling additive of claim 1 wherein themolecular weight of said polymer is in the range of from about 300,000to about 1.5 million and the molecular weight of said homopolymer is inthe range of from about 900,000 to about 1.5 million.
 13. A well cementcomposition comprising: a hydraulic cement; water present in an amountsufficient to form a slurry; and a viscosifying and fluid losscontrolling additive comprising a mixture of a polymer comprised of atleast one monomer which is calcium tolerant, anionic and disperses basiccement slurries, at least one monomer which hydrolyzes in basic cementslurries to generate anionic carboxylate groups that bind with calciumand viscosify the slurries and at least one monomer which generatesnon-ionic pendant groups on the polymer upon hydrolyzing in basic cementslurries to prevent polymer precipitation and a homopolymer of a monomerwhich hydrolyzes in basic cement slurries to generate anioniccarboxylate groups that bind with calcium, viscosity the slurries andprevent settling in the slurries.
 14. The cement composition of claim 13wherein said hydraulic cement is selected from the group consisting ofPortland cements, pozzolana cements, gypsum cements, aluminous cementsand silica cements.
 15. The cement composition of claim 13 wherein saidhydraulic cement is Portland cement.
 16. The cement composition of claim13 wherein said water is selected from the group consisting of freshwater, unsaturated salt solutions and saturated salt solutions.
 17. Thecement composition of claim 13 wherein said water is present in saidcomposition in an amount in the range of from about 38% to about 70% byweight of said hydraulic cement therein.
 18. The cement composition ofclaim 13 wherein said monomer in said viscosifying and fluid losscontrolling additive which is calcium tolerant, anionic and dispersesbasic cement slurries is selected from the group consisting of2-acrylamido-2-methylpropane sulfonic acid and its salts; vinylsulfonate, allylsulfonate and 3-allyloxy-2-hydroxy-1-propane sulfonicacid and its salts.
 19. The cement composition of claim 13 wherein saidmonomer in said viscosifying and fluid loss controlling additive whichis calcium tolerant, anionic and disperses basic cement slurries is2-acrylamido-2-methylpropane sulfonic acid or its salts.
 20. The cementcomposition of claim 13 wherein said monomer in said viscosifying andfluid loss controlling additive which hydrolyzes in basic cementslurries to generate anionic carboxylate groups which bind with calciumand viscosify the slurries is selected from the group consisting ofacrylonitrile acrylamide, N,N-dialkylacrylamide wherein the alkyl groupis selected from C₁ to C₆ alkyl groups, N-vinylpyrrolidone,2-acrylamido-2-methylpropane sulfonic acid and its salts, alkylacrylatewherein the alkyl group is selected from C₁ to C₆ alkyl groups andalkylmethylacrylate wherein the alkyl group is selected from C₁ to C₆alkyl groups.
 21. The cement composition of claim 13 wherein saidmonomer in said viscosifying and fluid loss controlling additive whichhydrolyzes in basic cement slurries to generate anionic carboxylategroups which bind with calcium and viscosify the slurries is acrylamide.22. The cement composition of claim 13 wherein said monomer in saidviscosifying and fluid loss controlling additive which generatesnon-ionic pendant groups on the polymer upon hydrolyzing in basic cementslurries to prevent polymer precipitation is selected from the groupconsisting of N-alkyl-N-vinyl-acetamide wherein the alkyl group isselected from C₁ to C₆ alkyl groups, allyl glycidyl ether andvinylacetate.
 23. The cement composition of claim 13 wherein saidmonomer in said viscosifying and fluid loss controlling additive whichgenerates non-ionic pendant groups on the polymer upon hydrolyzing inbasic cement slurries to prevent polymer precipitation isN-alkyl-N-vinyl-acetamide.
 24. The cement composition of claim 13wherein said monomer in said viscosifying and fluid loss controllingadditive which is calcium tolerant, anionic and disperses basic cementslurries is present in said polymer in an amount in the range of fromabout 30% to about 60% by weight of said polymer.
 25. The cementcomposition of claim 13 wherein said monomer in said viscosifying andfluid loss controlling additive which hydrolyzes in basic cementslurries to generate anionic carboxylate groups which bind with calciumand viscosify the slurries is present in said polymer in an amount inthe range of from about 20% to 60% by weight of said polymer.
 26. Thecement composition of claim 13 wherein said monomer in said viscosifyingand fluid loss controlling additive which generates non-ionic pendantgroups on the polymer upon hydrolyzing in basic cement slurries toprevent polymer precipitation is present in said polymer in an amount inthe range of from about 0 to 40% by weight of said polymer.
 27. Thecement composition of claim 13 wherein said polymer in said viscosifyingand fluid loss controlling additive is present in the mixture of saidpolymer with said homopolymer in an amount in the range of from about50% to about 95% by weight of said mixture and said homopolymer ispresent therein in an amount in the range of from about 5% to about 50%by weight of said mixture.
 28. The cement composition of claim 13wherein said polymer in said viscosifying and fluid loss controllingadditive has a molecular weight in the range of from about 300,000 toabout 1.5 million and said homopolymer therein has a molecular weight inthe range of from about 900,000 to about 1.5 million.
 29. The cementcomposition of claim 13 wherein said viscosifying and fluid losscontrolling additive is present in said cement composition in an amountin the range of from about 0.2% to about 7% by weight of hydrauliccement therein.
 30. The cement composition of claim 13 wherein saidviscosifying and fluid loss controlling additive is present in saidcomposition in an amount of about 2% by weight of hydraulic cementtherein.
 31. A well cement composition comprising: a hydraulic cement;water present in an amount sufficient to form a slurry; and aviscosifying and fluid loss controlling additive comprising a polymerhaving a molecular weight of about 500,000 comprised of2-acrylamido-2-methylpropane sulfonic acid, acrylamide andN-alkyl-N-vinyl-acetamide monomers, said 2-acrylamido-2-methyl propanesulfonic acid monomer being present in said polymer in an amount in therange of from about 40% to about 50% by weight of said polymer, saidacrylamide monomer being present in said polymer in an amount in therange of from about 30% to about 40% by weight of said polymer and saidN-alkyl-N-vinyl-acetamide being present in an amount in the range offrom about 10% to about 20% by weight of said polymer and a homopolymerof acrylamide having a molecular weight of about 1 million.
 32. Thecement composition of claim 31 wherein said hydraulic cement is selectedfrom the group consisting of Portland cements, pozzolana cements, gypsumcements, aluminous cements and silica cements.
 33. The cementcomposition of claim 31 wherein said hydraulic cement is Portlandcement.
 34. The cement composition of claim 31 wherein said water isselected from the group consisting of fresh water, unsaturated saltsolutions and saturated salt solutions.
 35. The cement composition ofclaim 31 wherein said water is present in said composition in an amountin the range of from about 38% to about 70% by weight of said hydrauliccement therein.
 36. The cement composition of claim 31 wherein saidpolymer in said viscosifying and fluid loss controlling additive ispresent in the mixture of said polymer with said homopolymer in anamount in the range of from about 50% to about 95% by weight of saidmixture and said homopolymer is present therein in an amount in therange of from about 5% to about 50% by weight of said mixture.